The purpose of validating a conceptual schema is to guarantee that it properly reflects what the user needs from an application. This task is not fully formalizable, so it is desirable to provide the designer with a set of tools that assist him or her in the validation process. A conceptual schema specifies the relevant information about the domain, and how this information changes as a result of operations. In this sense, we propose an approach to validate a UML conceptual schema by simulating the execution of the operations defined in it, so that the designer can check both that the schema is correctly defined and that it satisfies the requirements.